Re-thinking the Variability of Wind and Solar Power

As utilities, investors, and researchers gear up for next week's WindPower 2008 conference in Houston, the nascent wind industry's potential is on many people's minds. Just how much of future electricity demand will wind be able to meet? And what will the industry need to get there?

TreeHugger readers are probably already aware of a recent DOE study that projected wind supplying 20 percent of U.S. electricity by 2030. That's about as much electricity as nuclear power contributes to the U.S. mix today.

But bringing that much wind online means overcoming many hurdles.

One of these hurdles is rethinking how wind (and, for that matter, solar) installations are evaluated financially. A big drawback investors and utilities have found with these renewable energy sources is that they are "variable." Simply put: they can't generate electricity when the sun's not shining or the wind isn't blowing.

In the past, utilities believed that they had to compensate for this variability by installing more fossil-fueled power plants. The more wind or solar on the grid, the thinking went, the greater the need for reliable backup generating facilities.

Recently, however, researchers have started to question this notion. Simulations and models have shown that connecting geographically dispersed wind and solar sites can greatly decrease the variability of the whole system and reduce the need for backup power plants.

The latest two studies on the topic out of Rocky Mountain Institute (RMI) contend that utility managers need to think about their renewable energy investments as a portfolio.

"No person would invest in just one stock," says Lena Hansen, a senior consultant with RMI's Energy & Resources Team. In the financial markets, people forego the huge risks and potentially large gains of owning shares of one company for the reduced risk and smaller rates of return of owning shares in multiple companies.

Hansen argues that the same should go for utilities investing in variable energy sources like wind and solar. "By diversifying the portfolio of sites, you mitigate variability," she says.

The trick is to balance risks and rewards. Sites that tend to generate wind at different hours of the day can offset each other's periods of low production or inactivity. Similarly, solar installations could be sited at different longitudes so that collectively they are catching sun over a greater number of hours each day.

The real payoff, though, is combining wind and solar resources into one portfolio. In their first simulation of 43 sites throughout the upper Midwest, Hansen and her colleagues found that an optimal combination of solar and wind sites could reduce variability in the whole system by 55 percent. Surprisingly, those results bore out whether they looked at all 43 sites together or as few as six optimally selected ones.

In a follow-up study being presented at WindPower 2008, the RMI team enlarged their simulation outside the upper Midwest to include all the Great Plains states. They also looked at a longer time series (three years instead of one) for the upper Midwest study area.

The findings? Expanding the study area decreases the variability of a wind and solar portfolio even more than in the original simulation. The results also seem to hold true even for a longer time series.

Both findings further validate the value of a diverse portfolio of renewables and give credence to an old adage: the whole is greater than the sum of its parts.

You can read the full studies here.

Image Credit::© / Nikada

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